Potato and Carrot as Assessment

Abbey Grammar School AS BIOLOGY PRACTICAL ASSESSMENT Comparing the water potential of potato and carrot tissue.

Name: _____________________

Plant storage organs, such as potatoes and carrots, contain stores of carbohydrates, which may be soluble sugars or insoluble starch. Samples of tissue are immersed in a range of external solutions of different strengths. The solution that induces neither an increase nor a decrease in the volume or mass of the tissue has the same water potential as that of the cells in the tissue.

Section A: PREDICTION.

1. How do you predict the estimated water potential for potato will compare with that for carrot tissue? I predict that the carrot samples will have a more negative water potential than the potato samples. Osmosis is the movement of water from an area of higher water potential to an area of lower water potential through a semi permeable membrane. The carbohydrates stored within carrots are soluble sugars, while the carbohydrates in potatoes are insoluble starch; this difference in carbohydrates is what I base my prediction on. I believe that they will have more negative water potentials due to the soluble sugars in carrot cells meaning more water molecules can travel into the cells, making them turgid. Consequently, I believe that the carrot samples will have a greater mass than the potato samples.

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Section B: Recording of Results. Tabulate your results below and use appropriate graphical technique to present your results. Graph these results by plotting the average percentage change in mass against the molarity of the sucrose solutions. Use this data to find out the average water potential of the potato tissue.

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The results below were obtained from a similar experiment using carrot discs instead of potato discs. Three replicates were included for each bathing solution. Graph these results by plotting the average percentage change in mass against the molarity of the sucrose solutions. Use this data to find out the average water potential of the carrot tissue.

Table1: Change in mass of carrot tissue immersed in different concentrations of sucrose solution. Molarity of solution Water

0.2m

0.4m

0.6m

Mass before/g 0.70 0.81 0.65 0.59 0.65 0.59 0.55 0.64 0.65 0.63 0.65 0.55

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Mass after/g 0.80 0.96 0.73 0.68 0.71 0.67 0.58 0.67 0.69 0.59 0.60 0.56

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Section C: Interpretation of results. 1. State the estimated average water potentials of the two tissues. From my collated results we can see that the estimated value for the water potential of potato is -540kPa in 0.2moldm-3 of sucrose solution and the estimated value for the water potential of the carrot is -1400kPa in a sucrose solution of 0.5moldm-3. These results were obtained by plotting a calibration curve on graph paper and drawing lines from each of the values until they met the ‘curve’ on the graph. The point at which the line met curve on the y-axis was then used as the estimated average water potential.

2. Do the estimates of the average water potentials of the two tissues agree with the predicted outcome of this investigation? The water potential of distilled water is 0kPa. From my collated results we can see that the potato sample water potential was far less negative than the carrot samples at -540kPa and -1400kPa respectively. My predictions were correct.

3. Use your knowledge of solute and water potential of plant cells to explain the results of this investigation. In your explanations consider how the type of substrate stored influences the water potential. Water moves across cell membranes through special protein-lined channels, and if the total concentration of all dissolved solutes is not in equilibrium, there will be net movement of water molecules into or out of the cell. Whether there is net movement of water into or out of the cell depends on whether the cell’s environment is isotonic, hypotonic, or hypertonic. Hypertonic describes a solution that has a lower water potential (or more concentrated) than another solution. Hypotonic describes a solution that has a higher water potential (or

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Abbey Grammar School more dilute) than another solution. Isotonic describes a solution that has the same concentrations. Osmosis is the movement of water from an area of high water potential to an area of low water potential, through a semi permeable membrane. Water is the potential energy of water per unit volume relative to pure water in reference conditions. Pressure potential is calculated when molecules of water collide with the cell membranes. This value is then used to generate water potential. Pressure potential is only present in plant cells as a cell wall is needed for this to exist. Animal cells lack this cell structure. This is summarised in the equation; Ψcell = Ψpressure + ΨSolute. Within the carrot and potato, solutes, such as glucose, restrict the movement of water. When solutes and water molecules meet, hydration shells are formed due to the attractive forces. The molecule of water attaches itself to the solute, rendering it unable to leave the cell. Pure water has a water potential of 0 due to the lack of any solutes in the solution. If a cell has a water potential which is closer to 0 it is described as having a more positive water potential. Carrots store carbohydrates in the form of sucrose which is soluble. Sucrose is the product of alpha glucose which has bonded with fructose. Potatoes store glucose in the form of starch, an insoluble sugar. The carrot samples had very negative water potentials at -1400kPa. This can be explained by the fact that the soluble sugar sucrose (stored in carrots) forms hydration shells easily with water resulting in more negative values of water potentials. The values for the potato sample’s water potential back up this theory. Potatoes contain an insoluble sugar and therefore formed less hydration shells resulting in higher water potentials; values which were less negative and therefore closer in value to 0 (pure water).

Section D: Evaluation of the experimental Design, 1. Comment on the measurements made: A top-pan balance was used to measure masses to two decimal places of accuracy. This was extremely precise as samples were measured to the closest 0.01g. All measurements in mass were further converted to percentages so that they could be compared and accurately graphed.

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2. Could the results have been affected by other factors not under investigation? Our drying method could have been affected by human error. We had no way of specifying a value of applied force when drying our samples. Some may have been more dried out than others. Using a syringe i was able to ensure that the concentrations and volumes were equal to a high degree of accuracy.

3. Comment on the validity of the experimental design and procedures; Different potatoes may have differing percentages of starch, even within a single potato. An increased level of starch could significantly alter water potential results, making it less negative than other samples. All six cylinders of potato were taken from the same potato, meaning out results would be accurate and valid. Non uniform samples were discarded to increase the validity of our collated results and graphs. Test tubes were covered with stoppers to ensure that no air moisture would enter. The tubes were kept at the same temperature and conditions for 24 hours.

2. Using the class data assess the variation shown within the replicates of the percentage change in mass at each molarity for the potato discs.

Overall, I believe that there was very little variation was shown within the replicates. Out of eight groups, only one group's results proved anomalous. Group 8's results may have been anomalous due to several factors. Although a standardised drying method was used, the applied force on each cylinder was extremely hard to gauge, causing some cylinders to inevitably be more dry than others and therefore affecting their masses. The potato Group 8 used may have had a greater concentration of starch, increasing water potential and final values. If I were to repeat the experiment, discarding Group 8's results would be beneficial to increasing the validity of our graphs.

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5. Comment on the reliability of the estimates of the water potential made. Our results were reliable for many reasons; The use of skewers ensured that surface area was not an issue, additionally, we created eight replicates meaning our results were accurate and reliable. Each solution was stirred before use and each pupil had ample time to complete their experiment. Despite this, i feel that group 8’s results were anomalous and should have been discarded for increased reliability and a more accurate graph. Overall, due to the replication and consistency of the results, i believe that the experiment was quite reliable.

Table 2 Relationship between molarity and water potential of sucrose solutions

Molarity (mol dm) 0.05 0.10 0.15 0.20 0.25 0.30

Water potential kPa -130 -260 -410 -540 -680 -860

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Abbey Grammar School 0.35 0.40 0.45 0.50 0.55 0.60

-970 -1120 -1280 -1450 -1620 -1800

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